Position your oven rack in the lower third of the oven and preheat to 350°F (175°C). Lightly grease only the bottom of a 9-inch round springform pan using non-stick baking spray and set it aside. In a double boiler set over medium-low heat, melt the butter and chopped chocolate together, stirring frequently until the mixture is smooth and glossy. Once melted, whisk in the cocoa powder and espresso powder until well combined, then set aside to cool slightly.

To make a double boiler, fill a saucepan with about 2 to 3 inches of water and place a heat-safe bowl over the top, ensuring the bottom of the bowl does not touch the water. Gently heat without letting the water boil or simmer, stirring until the chocolate and butter are completely melted. Separate the eggs, placing the yolks in a medium mixing bowl along with the vanilla extract and whisking until smooth. Transfer the egg whites to the bowl of a stand mixer fitted with a whisk attachment.

Add the cream of tartar to the egg whites, then beat on medium-low speed until the whites start to foam and create bubbles. Gradually increase the mixer speed to medium-high and continue beating until soft peaks form. Keeping the mixer on medium-high, slowly add the granulated sugar one tablespoon at a time, beating until stiff, glossy peaks form.

Gently fold about one-third of the whipped egg whites into the yolk mixture to lighten it. Then fold in the melted chocolate and butter mixture until incorporated. Carefully fold in the remaining whipped egg whites in three separate additions, using a large spatula and folding just until no streaks of egg white or chocolate remain. Take care not to overmix, as this will deflate the egg whites and affect the cake’s airy texture.

Pour the finished batter into the prepared springform pan and use an offset spatula to smooth the surface. Bake the cake for 30 to 35 minutes, or until the top has puffed up and cracked slightly, revealing a moist center beneath the cracks. The surface should appear smooth and matte when done. Let the cake cool in the pan for about 20 minutes. Then, run a knife around the edge to loosen the sides and carefully release the springform ring. Expect the cake to deflate slightly as it cools—this is completely normal and part of its signature texture.